An extended nip press is a press for papermaking in a papermaking machine which increases the residence time of a web during passage of the web through a pressing nip. Typically, a nip is thought of as the narrow region about the line of co-tangency when two rolls with aligned axes are brought tangent to one another. The nip between rolls has classically been used in papermaking to remove water and to compress the fibers in the paper web into a smooth surface. Within a papermaking machine it has been found to be desirable to increase the area of the nip and so subject the paper web to a somewhat lower pressure, more uniform pressing in which more heat transfer between the roll and the paper can take place. In some circumstances, the nip can be extended by coating the surface of one or both of the rolls with a compliant material. The use of a compliant roller allows for nips somewhat greater than an inch in length along the direction of the web through the nip.
The desire for even longer nips led to the development of the so-called Extended Nip Press (ENP). The ENP employs a backing roll and an elongated shoe which has a concave surface which matches the convex surface of the backing roll and cooperates with the roll over a length on the order of ten inches along the direction of travel of the paper web. The shoe, which forms one-half of the nip, is stationary. Without further modification, a paper web moving through the nip formed between the shoe and the backing roll would experience unacceptable rubbing on the non-moving shoe. To overcome this problem, a bearing blanket forming a cylindrical tube is slidably disposed over the shoe and around the support shaft on which the shoe is mounted. Friction between the bearing blanket and the shoe is greatly reduced due to the presence of an oil film which is supplied between the nip and the backside of the blanket. The blanket thus freely slides on an oil film on the shoe.
To aid in the drying or pressing of the paper web, a felt or blanket often underlies and supports the paper web as it transits the extended nip between the backing roll and the bearing blanket on the shoe. The backing roll, the paper web and the web support blanket, if present, are frictionally engaged and in turn engage the upper surface of the bearing blanket, causing it to slide over the shoe and to rotate about the shoe and its support shaft.
On one type of extended nip press the bearing blanket is tensioned and supported by rolls which result in open ends to the cylindrical blanket. The open end of the bearing blanket causes a problem with oil contamination of the web. There is a tendency for lubricating oil disposed between the shoe and the blanket to creep around the edges of the blanket so that the press felt and the paper web become contaminated.
Further, such oil tends to atomize and subsequently settle as an oily film on ancillary equipment. Such oily film causes a potential hazard in the form of slippery walk ways and access ladders. It also poses a potential fire hazard in that the atomized oil droplets have a relatively low flash point. The solution to this problem has been to seal the ends of the blanket to two rotating heads which are mounted on the shoe support shaft. The edges of the bearing blanket are extended beyond the shoe where they are free to take on a circular cross section which may be sealed or joined to the disc shaped rotating heads.
The cross-section of the blanket at the heads is circular, but the cross-section taken through the nip between the shoe and the backing roll has an appearance somewhat similar to an apple in cross-section. The shape of the blanket is circular, with a concave, dimpled surface where the stem of an apple would be. Thus, as the press blanket moves through the nap, it is elastically deformed. More particularly, between the ends of the nip and the ends of the blanket, the material of the blanket forms a compound curve as the blanket transits from the shape of the nip to the circular ends of the blanket where they are attached to the rotating head assemblies. As a result of the compound curve flexure in the blanket the blanket must be periodically replaced.
A typical papermaking machine of which the extended nip press forms a part may produce over half a million square feet of paper an hour. Thus, down time is costly and the method of clamping the blanket to the rotating head assemblies must be compatible with rapid and precise replacement of the blanket.
What is needed is a blanket-to-head clamping device of improved simplicity and performance.